In gas phase polymerization, a gaseous stream containing one or more monomers is passed through a fluidized bed under reactive conditions in the presence of a catalyst. A polymer product is withdrawn from the reactor. Fresh monomer is introduced to the reactor to replace the removed polymer product, and any unreacted monomer is recycled back to the reactor.
Process upsets in an ancillary system upstream and/or downstream of the reactor frequently require the polymerization to be shutdown or “killed.” Shutdown procedures are often accompanied with a buildup of catalyst and polymer on the walls of the reactor, which is known as “sheeting.” Another common problem is the buildup of catalyst and polymer on the internal distribution plate, injection nozzle(s), and/or product discharge nozzle(s), which is known as “plugging” or “plate fouling.”
Typical kill procedures require the reactor to be opened, purged of hydrocarbons, emptied of polymer and catalyst particles, cleaned, and reloaded with the removed bed or a new bed to provide a “seedbed” of polymer. This process is time consuming, expensive, and allows impurities, such as moisture and air, to enter the reactor. Such impurities necessitate another time consuming procedure to remove, which typically involves a nitrogen purge to reduce the impurity levels to less than 10 ppm, before restarting the reactor. There is a need, therefore, for improved methods for shutting down and restarting polymerization in a gas phase polymerization reactor.